Process for continuously pickling metallic strip material



Dec. 16, 1969 P. K NIEDNE R' ET AL 3,434,233

PROCESS FOR CONTINUOUSLY PICKLING METALLIC STRIP MATERIAL Filed March 21, 1966 Willi/lilo. III/0711]!!! IIIIIIIIIIIIO.

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ATTORNEY United States Patent Int. Cl. B0sb5/02, 7/02 US. Cl. 1349 2 Claims ABSTRACT OF THE DISCLOSURE Steel strip is pickled at a greater production rate than hitherto obtained with an acid tank of given size. Before being submerged in the pickling acid, the strip is passed through a scale breaker and is then heated, preferably by means of jets of steam, before being passed into the acid tank where the oxide scale is removed. The steam-jet heating speeds the pickling action in two ways: (1) the steel strip is already but when it enters the acid tank and (2) the steam enters microscopic cracks in the oxide layers more readily than a liquid would, condenses there, and serves as a carrier for the pickling acid to diffuse readily to the bottoms of the cracks.

This invention relates to the art of pickling metallic bodies to remove scale and the like therefrom, and more particularly to a process for continuously pickling metallic strip material at a greater rate of speed than has heretofore been possible.

In continuously operated pickling equipment for elongated metallic bodies, particularly wide strips, the strip is usually introduced into a heated storage container filled with water and thereafter passed continuously into the pickling bath. The entrance temperature of the body or strip to be heated is usually between 50 C. and 60 C. With this process, it usually takes a substantial period of time until the temperature of the body reaches the reaction temperature in the pickling tank, the latter usually being operated at temperatures above 100 C.

The primary object of the present invention is to reduce the time required to pickle elongated strip material, thereby facilitating the use of shorter pickling tanks. As will be seen, this object is accomplished by preheating the elongated strip to an elevated temperature before introduction thereof into the pickling bath and by conditioning the strip such that the pickling action (i.e., etching) occurs more rapidly.

In accordance with the invention, elongated metal bodies, particularly wide strips to be continuously pickled, are preheated prior to passage into the pickling bath by exposing the surface of the strip to pressurized hot steam. The effect of the hot steam is increased by passing the elongated bodies through a scale breaker prior to exposure to the hot steam.

The novel process of the invention results in high heat transfer COEffiClCIltS, thereby providing rapid heating of the strip. Since, due to the effect of the scale breaker, the scale is broken or loosened, the pickling acid finds easier access to the more readily dissoluble scale layers under the less soluble outer oxide skin. It has been found that hot steam blown directly against the strip condenses in the cracks formed by breaking the scale. The cracks so formed are so small that a liquid can enter them only very slowly. The condensation product of the steam, however, fills the cracks and has a temperature near the boiling point of water. Upon passage of the strip containing a residual amount of condensation product into the pickling bath,

the pickling acid readily reaches the reaction points by diffusion, thereby enhancing the pickling process.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a cross-sectional view of the scaled surfacle of metallic strip material, for example, at an enlarged sca e;

FIG. 2 schematically illustrates the continuous pickling apparatus and arrangement of the invention; and

FIG. 3 schematically illustrates the heat treatment of the metallic body by means of hot steam projected from nozzles.

With reference now to the drawings, and particularly to FIG. 2, an elongated body A comprising metallic strip material, and preferably steel strip, is reeled off a reel 1 and passed through a scale breaker 10 and a pre-treatment chamber 5. From the chamber 5, the strip material passes into the pickling tank 2 containing a chemical substance which will react with or attack the surface of the strip A. After passage through the pickling tank 2, the strip is cold rinsed in chamber 3, hot rinsed in chamber 4, and thereafter dried in chamber 6. Drying air is admitted to the drying chamber 6 from a blower 7, the air passing through a heat exchanger 8 before passage into the drying chamber 6. The pickled and dried strip is then wound upon the reel 9.

With reference, now, to FIG. 3, details of the pre-treatment chamber 5 are shown. The chamber 5 is sealed except for slots at 11 through which the strip A passes. By means of the nozzles 12 above and below the strip A, hot steam at considerable pressure is blown against the strip. The strip, being cooler than the steam, causes the steam to condense on its surface. The accumulated condensation which runs off the surface of the strip is collected in a sump 13 and drain 14. In order to enhance condensation of excess steam, cooling pipes 15 are provided within the container 5.

In FIG. 1, the base metal of strip A, for example, is designated by the reference numeral 16. The various scale layers are denoted by the reference numerals 17, 18 and 19. The cracks produced by the scale breaker 10- are denoted by the reference numeral 20. FIG. 1 also shows one of the steam nozzles 12 and indicates how the steam condenses to water 20 at the bottomsof the cracks C produced by the scale breaker 10. Upon passage of the strip into the pickling tank 2, a pickling solution, usually acid, more easily reaches the metal surface due to a diffusion effect. As was mentioned above, liquid can enter the cracks only very slowly. The condensed steam, having already entered the cracks before the strip enters the pickling tank 2, mixes with the pickling solution and, in effect, acts as a carrier to get the pickling solution quickly to the lower scale layers where it is most effective. This diffusion efiect and the elevated temperature of the strip cooperate to thereby substantially reduce the pickling time and also reduce the required length of the tank 2 for a given strip speed.

Although the invention has been shown in connection. with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. A method for pickling metallic bodies, which comprises the steps of breaking scale on the surface of a metallic body so as to form cracks therein, thereafter subjecting the body to steam whereby the body becomes heated and steam which condenses on the surface of the body flows into said cracks, and thereafter, while scale remains on said body, submerging the body in a pickling solution, said method being further characterized in that the metallic body comprises a continuously moving strip material which passes from a scale breaker to steam nozzles and thence into a pickling tank, the pickling solution having a temperature in excess of 100 C., with the steam heating the body to a temperature above 60 C. before it is submerged in the pickling solution, and with steam being applied to the continuously moving strip in a container having therein a cooling means to cause condensation of steam, thereby effecting diffusion of condensing steam into said cracks to fill the same with condensed steam at a temperature near the boiling point of Water.

References Cited UNITED H) MORRIS O. WOLK,

Primary Examiner JOSEPH T. ZATARGA, Assistant Examiner US. Cl. X.R. 

